Cryogenic vacuum pumps (cryopumps) are widely used in high vacuum applications. Cryopumps remove gases from a vacuum chamber by cooling the gases and then binding the gases to cold surfaces inside the pump. Cryocondensation, cryosorption and cryotrapping are the basic mechanisms that can be involved in the operation of a cryopump. In cryocondensation, gas molecules are condensed on previously condensed gas molecules. Thick layers of condensate can be formed, thereby pumping large quantities of gas.
Cryopumps are widely used for applications where contamination by non-process gases such as hydrocarbons must be avoided. Cryopumps typically use a closed loop helium refrigerator. Refrigeration is produced in a first stage operating at 50 to 80 degrees K and a second stage at 10 to 20 degrees K. Conducting metal surfaces called cryoarrays are attached to the refrigerator stages and are cooled thereby. Easily condensed gases, such as water vapor, argon, nitrogen and oxygen, are pumped by cryocondensation on the first and second stage cryoarrays. However, the lowest temperature achievable in a refrigerator cooled cryopump is sufficiently high (about 10.degree. Kelvin) that not all gases normally present in a vacuum system can be pumped by cryocondensation. The gases which are difficult to condense, such as hydrogen, helium and neon, must be pumped by cryosorption. For this purpose, a sorbent material such as activated charcoal is attached to the second stage cryoarray. Further, only relatively low amounts of gas can be pumped by cryosorption, as only a thin layer (up to about 5 monolayers) can be formed on the surfaces of the sorbent material.
Cryopump applications are requiring the first stage of the cryopump to be maintained at a particular and uniform temperature. One cryopump attempts to meet this requirement by providing a heater in thermal contact with the first stage cryoarray. The heater works against the action of the refrigeration source by adding heat to the first stage to maintain a predetermined temperature, as measured by a temperature sensor.
The use of heaters to maintain the temperature of the first stage array detracts from the overall efficiency of cryopumps in several ways. First, the heat applied to the first stage cryoarray adds to the cooling load of the refrigeration source. In addition, heat added to the first stage can degrade the operational conditions of the second stage by raising its temperature, thereby reducing the pump's overall efficiency. Finally, the power used to operate the heater adds to the power needed to operate the entire cryopump system, thereby making the system less energy efficient.
Ultimately there is a limit to the amount of gases that can be pumped by a cryopump, necessitating a need for the pump to be regenerated. The usual process for regeneration involves decoupling the cryopump system from the chamber it is pumping, deactivating the refrigerating system, and allowing the cold surfaces within the pump to warm and release captured gases. Once the gases are released and vented from the vacuum system, a secondary roughing pump is used to restart the vacuum pump-down. After a suitable vacuum level is achieved, the cryopump is restarted by reactivating the refrigerator to recool the internal surfaces so that the internal mechanism can recommence the normal operation of cryopumping. This regeneration process of boiling off and venting the contaminant gases and reestablishing normal vacuum conditions for cryopump operation usually takes several hours, and the time consumed this way prevents the use of the pump for its intended purposes.
U.S. Pat. No. 4,763,483, although primarily concerned with pump-down concept, discloses the regeneration of the cryopump through the movement of the cooling segments of each stage away from the cryoarrays of each stage while the refrigeration system continues to function. The pump is then brought on-line by starting the first stage and bringing it to a certain level of operation at which time the second stage is reconnected and made operational.
It is an object of this invention to provide a cryopump having an isolation mechanism for temporarily breaking the thermal contact between the refrigerating system and one or more of the arrays of the cryopump. It is a further object of this invention to use a thermal isolation mechanism to perform a partial regeneration by only regenerating the condensed gases on the second stage array of the cryopump. It is yet a still further object of this invention to be able to do a partial or full regeneration by choice using the techniques of this invention. It is yet another object of this invention to provide an isolation mechanism for automatic and noninvasive temperature control of the first array of a cryopump.